FIG. 3 is a cross-sectional side view of a prior art semiconductor laser device which is disclosed in Vol. 3 of prescript of the 49th Study Meeting of Applied Physics, p. 834. In FIG. 3, a semiconductor substrate 11 of a first conductivity type is provided at the bottom of the device. A first conductivity type cladding layer 12 and an undoped active layer 13 are successively provided on the substrate 11. On the layer 13, there is provided a second conductivity type semiconductor layer having an energy band gap wider than the active layer 13 (hereinafter referred to as "a barrier layer", for it acts as a potential barrier against carriers injected into the active layer 13), and a second conductivity type semiconductor layer 15 with an energy band gap as wide as the active layer 13 (hereinafter referred to as "an absorption layer", for it absorbs the light). Reference numeral 16 designates a diffraction grating produced on the surface of the absorption layer 15. On the layer 15, a second conductivity type cladding layer 17 is provided.
The operating principle of this device will be described.
According to Journal of Applied Physics, vol. 43, pp. 2327 to 2335 (1972), when the reflectance n and the gain .alpha. have a periodicity in the cavity length direction of the laser resonator (hereinafter referred to as "Z direction"), as represented by; EQU n(z)=n.sub.0 +n.sub.1 .multidot.cos (2.TM.z/.LAMBDA.) EQU .alpha.(z)=.alpha..sub.0 +.alpha..sub.1 .multidot.cos (2.TM.z/.LAMBDA.)
(herein .LAMBDA. is a pitch of a diffraction grating), the coupling coefficient .kappa. is defined by; EQU .kappa.=n.sub.1 /.lambda.+i.multidot..alpha..sub.1 /2
where .lambda. is the oscillation wavelength and i is the imaginary number unit.
In a case where .pi.n.sub.1 /.lambda.&gt;&gt;.alpha..sub.1 /2, i.e., the refractive index controls the coupling coefficient .kappa., the device is called a refractive index coupling type distributed feedback semiconductor laser device. Herein, the laser device oscillates ordinarily at two wavelengths. Such laser device has problems such as noise generated by mode competition of two wavelengths and deterioration of signal waveforms by wavelength dispersion within optical fiber. In order to prevent these, a laser device which oscillates at a single wavelength is fabricated by using a increasing the reflectance of one facet of a laser resonator and lowering the reflectance of the other facet, or by shifting the phase of a diffraction grating by .pi. at the center portion of the resonator. In these methods, however, there are problems in the stability of oscillation at a single wavelength or in fabrication.
In a case where .pi.n.sub.1 /.lambda.&lt;&lt;.alpha..sub.1 /2, i.e., the gain controls the coupling coefficient .kappa., the device is called a gain coupling type distributed feedback semiconductor laser device. Herein, the device oscillates at a single wavelength only by coating both facets of the resonator to be of low reflectance, and is superior to the refractive index coupling type one in fabrication and stability of oscillation at a single wavelength.
FIG. 3 shows a gain coupling type laser device based upon the above described ideas. Herein, an active layer 13 has gain necessary for laser oscillation. An absorption layer 15 having an energy band gap as wide as that of the active layer 13 has a high absorption coefficient for a guided light wave, and furthermore a large periodic of absorption is produced inside the cavity because a diffraction grating is produced on the layer 15. As the absorption is only the inverse of the gain, a large periodic gain results, thereby providing a gain coupling type semiconductor laser device.
In the prior art semiconductor laser device, however, the refractive index of the absorption layer 15 and that of the second conductive type cladding layer 17 are different from each other, and this brings about the periodic refractive index. Thus, the condition of .pi.n.sub.1 /.lambda.&lt;&lt;.alpha..sub.1 / 2 is not satisfied/ and it is difficult to produce stable, single mode oscillations.